1. Field of the Invention
The present invention relates to an apparatus for transferring high frequency signals, and, more particularly, to an apparatus for transferring high frequency signals with reduced losses at desired frequencies and/or with attenuation at undesired frequencies.
2. Description of the Related Art
In the design of low cost microwave electronics, it can be very difficult to properly shield the electronics and to limit undesired electromagnetic emissions at harmonic frequencies of the fundamental transmission. The undesired electromagnetic emissions are tightly regulated by the U.S. federal government and by the European Union, and thus there may be a need to reduce the magnitude of such emissions. Doppler transceivers for motion sensors are one example of low cost microwave electronics that may need shielding and reducing of emissions.
Typically, exotic materials and techniques are used to contain the unwanted emissions within the enclosure or housing of the electronics. For example, it is known at Robert Bosch Corporation to provide the active electronic circuit on one side of a circuit board and to provide antenna arrays, to which the active electronics are communicatively coupled, on the other side of the same circuit board. However, difficulty is presented in transmitting the desired signal from one side of the circuit board to the other (e.g., from the circuit side to the antenna side) without significant signal loss in the desired frequency range.
U.S. Pat. No. 5,262,783 to Philpot, et al., depends on resonance for efficient power transfer in the desired frequency band of operation. A problem with this approach is that the use of resonance results in a relatively narrow passband.
In many known microwave and radio frequency transceiver devices it is necessary to transfer signals from one side of a multilayer circuit board to another side, and it would be desirable to make the transfer with a minimum loss in power. Traditionally, the transfer is accomplished by use of a microstrip to coaxial to microstrip transmission line transition. In order to maintain a 50 ohm impedance in such a transition, the diameter of metal clearance in the ground plane is typically required to be fairly large. This dimension is large enough is some cases to allow direct radiation from one region to another. In low cost high frequency devices, it can be very difficult to prevent the direct radiation of undesired harmonic emissions through this large aperture.
In known transitions, the approach has been to push the cutoff frequency as high as possible by making the structure, spacing, and dimensions constant from top to bottom, i.e., throughout the transition, to simulate the constant cross section of a coaxial cable.
Accordingly, what is neither anticipated nor obvious in view of the prior art is any apparatus that is as capable as the microstripline transmission line arrangement of the present invention at transferring high frequency signals with reduced losses at desired frequencies and with attenuation at undesired frequencies. Particularly, what is neither anticipated nor obvious in view of the prior art is a microstripline transmission line arrangement in which the spacing between the signal conductor and the grounded shield varies along the length of the signal conductor.